The invention relates to a piezoelectric material comprising a chiral compound and a polymer network having a smectic structure.
The invention also relates to a piezoelectric element comprising a piezoelectric material positioned between at least two electrodes, said piezoelectric material comprising a chiral compound and a polymer network having a smectic structure.
The invention further relates to a method of manufacturing a piezoelectric material by forming a polymer network from a starting material which comprises a liquid crystalline monomer material and a chiral compound.
Piezoelectric materials, in particular also synthetic resins such as polymeric materials, are suitable for use in many applications, such as in electromechanical components, generators and sensors, for example in microphones and speakers. Besides, use can be made of the pyroelectric effect and, in the ease of transparent materials, also of nonlinear optical properties.
The manufacture of a piezoelectric elastomer material is described by S. U. Vallerien et. al., in Makromol. Chem., Rapid Commun. 11, on pages 593-598 (1990). In this publication, polymers having mesogenic groups are mixed with a chiral compound to obtain spontaneous polarization in the polymeric material, after which the material is converted to an elastomer by means of thermal crosslinking from an isotropic phase. The resulting material may exhibit cholesteric and/or chiral smectic phases and ferroelectric properties in a multi-domain structure. If desired, an improved molecular orientation can be obtained by orienting the polymer molecules in a magnetic field during crosslinking.
The known material has the disadvantage that the piezoelectric effect is small, in particular, when a cholesteric phase is used. This is caused by the presence of a multi-domain structure as a result of which not all dipoles in the material are equally oriented, and by the fact that the mesogenic groups are partly situated in side chains of the polymer molecules, which is not very effective for obtaining a piezoelectric effect. In addition, the known material exhibits various phase transitions, so that the applicability is limited to narrow temperature ranges and the stability of a piezoelectric element manufactured with this known material is insufficient. Also the known method of manufacturing the piezoelectric material by thermally activating the crosslinking between the polymeric molecules causes any orientation of the molecules to be lost and leads to the formation of a loose network having a limited stability.